A gas turbine engine typically comprises a compressor, combustion system, and turbine, for the purpose of compressing air, mixing it with a fuel and igniting this mixture, and directing the resulting hot combustion gases through a turbine for creating propulsive thrust or rotational energy used for electrical generation. Turbine sections comprise a plurality of stages, where each stage includes a row of stationary airfoils followed by a row of rotating airfoils, where the row of stationary airfoils direct the flow of hot combustion gases onto the row of rotating airfoils at a preferred angle. The rotating airfoils of the turbine are driven by the pressure load from the hot combustion gases passing along the airfoil surface. While the rotating airfoils, or blades, are each individually attached to a turbine disk, which thereby allows each blade to move as necessary due to thermal gradients. However, stationary airfoils, or vanes, are often times manufactured in doublets or triplets, where two or three airfoils are interconnected by common platforms, which also serve as radial seals, such that hot combustion gases cannot leak out of the turbine and are directed towards the turbine blades, thereby increasing the overall turbine efficiency. An example of a prior art turbine vane doublet in accordance with this design is shown in FIG. 1. Turbine vane 10 includes a first airfoil 11, second airfoil 12, each of which are fixed to inner platform 13 and outer platform 14. A plurality of these vane doublets are assembled together in the engine case to form a stage of stationary airfoils.
While this arrangement is desired to prevent leakage of hot combustion gases into the region of turbine cooling air, often times adjacent turbine vane airfoils 11 and 12 have different operating temperatures and temperature gradients depending on the flow of hot combustion gases onto the vane airfoils. These temperature gradients are further affected by the cooling fluid passing through the airfoil section. As a result of this multi-vane configuration, the airfoils cannot respond as individual components thus creating high thermal stresses in vane assembly 10 resulting in severe cracking of airfoils 11 and 12 in a relatively short period of time.
What is needed is a turbine vane assembly arrangement that provides the sealing benefit of a multi-vane configuration while allowing individual airfoils to respond to varying thermal gradients.